Helical cytokines control multiple biological processes, ranging from host defense to development and body homeostasis. This family of ligands, consisting of Interleukin (IL-x) 2, 3, 4, 5, 6, 7, 9, 11, 12, 13, 15, 21, 23, thymic stromal lymphopoietin (TSLP), granulocyte factor (GM-CSF), granulocyte-macrophage colony stimulating factor (GM-CSF), erythropoietin (EPO), thrombopoietin (TPO), prolactin (PRL), growth hormone (GN), leukemia inhibitory factor (LIF), oncostatin-M (OSM), cardiotrophin-1 (CT-1), cardiotrophin-like cytokine (CLC), ciliary neurotrophic factor (CNTF), and leptin (OB), has a rich source of molecules with highly specific biological effects and important therapeutic potential.
The helical cytokine family is defined by a common three-dimensional structure consisting of an anti-parallel four helix bundle with a characteristic “up-up-down-down” topology. Bazan, J. F., Immunol. Today 11(10): 350-4 (1990), Rozwarski, D. A. et al., Structure 2(3): 159-73 (1994). Unfortunately, the lack of significant sequence homology has hampered the identification of novel members of this family by homology screens, and more recently, data mining. The cognate receptors, however, form a family of so-called type I cytokine receptors and share several structural motifs, including a cytokine receptor homology (CRH) domain with 2 pairs of conserved cysteine residues and a WSXWS sequence motif in the extracellular domain [Bazan, J. F., Proc. Natl. Acad. Sci. USA 87(18): 6934-8 (1990)], a single transmembrane domain and an intracellular domain without intrinsic enzymatic activity. These features allow for homology-based identification of novel receptors, which in turn can be used as tools to subsequently identify their ligands by a variety of different screening techniques. De Sauvage et al., Nature 369(6481): 533-8 (1994); Parrish-Novak J., et al., Nature 408(6808): 57-63 (2000); Lok, S. et al., Nature 369 (6481): 565-8 (1994).
Ligand binding induces homo- or heteromerization of at least two receptor subunits. In the former case, two identical receptor subunits form a homodimeric receptor that is sufficient for ligand binding and signaling [e.g., GH-R, de Vos, A. M. et al., Science 255(5042): 306-12 (1992)]. Heteromerization is induced when a ligand-specific α-chain forms a high affinity receptor in combination with a signal transducing β-chain. This β-chain is shared amongst several other α-chains, e.g., IL-3, IL-5, GM-CSF. Itoh, N. et al., Science 247 (4940): 324-7 (1990). In either instance, ligand binding to the receptor leads to activation of cytoplasmic tyrosine kinases of the Janus kinase (Jak) family, which associate with the receptor subunits through conserved box-1 and box-2 motifs within the membrane proximal part of the intracellular domain. Ihle, J. N., Nature 377(6550): 591-4 (1995). Jak activation leads to phosphorylation of cytoplasmic target proteins, in particular the intracellular domains of the receptors and members of the STAT protein family, which are recruited to phosphotyrosines on the receptor by means of their src-homology type 2 (SH2) domains. Ihle, J. H. Cell 84(3): 331-4 (1996); Ihle, J. H. et al., Stem Cells 15 (Suppl. 1): 105-11, discussion 112 (1997). Phosphorylation of STATs induces dimerization and translocation to the nucleus and results in specific activation of gene transcription. Darnell, J. E., Jr., Science 277 (5332): 1630-5 (1997). Seven STAT proteins are known to date (STATs 1, 2, 3, 4, 5a, 5b and 6). Analysis of animals deficient for STAT isoforms indicates that STATs mediate many of the specific effects of cytokines, Ihle J. N. Curr. Opin. Cell Biol. 13(2): 211-7 (2001), highlighting their key importance in cytokine receptor signaling. In addition to specific target gene regulation, and in combination with other signaling pathways activated by cytokine receptors, such as mitogen-activated protein kinase and phosphatidylinositol-3 kinase, STATs can contribute to anti-apoptotic and mitogenic signals upon activation Ihle, J. N. Nature 377(6550): 591-4 (1995).